Windows NT Systems Programming: Spring 2000

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Handling a List of Students: EX17a

Objectives of studying this exercise: (source code here)

• Use CObList collection object
• Navigating in the list
• Adding a custom, icon labeled toolbar
• Serializing a collection object

Collection Objects

• You remember writing these: Lists (linked lists and arrays), tables (hash tables, trees), and so on.
• Work well with templates for you can build them once and make them available in a library
• STL (Standard Template Library has vector, list, sets, maps, stack, etc MFC has CObList, CObArray which use polymorphism to overload I/O and other operator/functions.
• CObjList = ordered list of pointers to objects derived from CObject.
  basically this is implemented as a linked list which supports fast insertion/deletion at the ends and sequential search
• Advantages:
  • Grows dynamically
  • Allows mixed type objects (as long as they are derived from CObject) = polymorphism
  • can be serialized (can write entire collection to file/ network easily).
• Supports Iteration

Iteration

CObject*& GetNext( POSITION& rPosition );
// updates rPosition to next position
// can use pointer returned to modify object in list

CObject* GetNext( POSITION& rPosition ) const;

• Supports access to any position
  • POSITION FindIndex( int nIndex ) const
  • void SetAt( POSITION pos, CObject* newElement ); // overwrites object at position
  • POSITION InsertBefore( POSITION position, CObject* newElement );
    throw ( CMemoryException );
• See documentation for all member functions.

A List of Students

// in Student.h

typedef CTypedPtrList<CObList, CStudent*> CStudentList;
// A list of CStudent objects built on CObList - type safe use of template class CTypedPtrList

// in StuDoc.h
private:
    CStudentList m_studentList;

// for access to list (used by view object)
public:
    CStudentList* GetList() {
        return &m_studentList;
    }

View Accesses List

void CStudentView::OnInitialUpdate()
{
    m_pList = GetDocument()->GetList(); // remember the list
	CFormView::OnInitialUpdate();
}

void CStudentView::OnUpdate(CView* pSender, LPARAM lHint, CObject* pHint) 
{
    // called by OnInitialUpdate and by UpdateAllViews
    m_position = m_pList->GetHeadPosition();
    GetEntry(m_position); // initial data for view	
}

void CStudentView::GetEntry(POSITION position) 
{
    if (position) {
        CStudent* pStudent = m_pList->GetAt(position); 
        m_strName = pStudent->m_strName;
        m_nGrade = pStudent->m_nGrade;
    }
    else {
        ClearEntry();
    }
    UpdateData(FALSE); 
}

• Position is a type for denoting position in list (a kind of pointer)
• Gets a pointer to the object at the current position and sets the values which will go into the dialog (using DDX)
• Invokes DDX functions to load form data fields

Inserting new Student into document

void CStudentView::OnStudentIns() 
{
    InsertEntry(m_position);
    GetDocument()->SetModifiedFlag();
    GetDocument()->UpdateAllViews(this); 
	
}
void CStudentView::InsertEntry(POSITION position)
{
    if (UpdateData(TRUE)) { 
        // UpdateData returns FALSE if it detects a user error
        CStudent* pStudent = new CStudent; 
        pStudent->m_strName = m_strName;
        pStudent->m_nGrade = m_nGrade;
        m_position = m_pList->InsertAfter(m_position, pStudent);
    }
}

• Message handler for insert message (either from menu or toolbar)
• Document is marked as modified (so it will warn when exiting application DO YOU WANT TO SAVE?
• Update all views but this one. (in case there are multiple views - not needed for this program)
• UpdateData(TRUE) invokes DDX to move data from form to VIew object data members
• Create a new Student object to insert in list
• Insert after the current position and make new current position

Deleting a Student from the List

void CStudentView::OnStudentDel() 
{
    // deletes current entry and positions to next one or head
    POSITION pos;
    if ((pos = m_position) != NULL) { // how good is your c++
        m_pList->GetNext(pos); // iterate to next position (updates pos)
        if (pos == NULL) { // at end - set position to head
            pos = m_pList->GetHeadPosition();
            if (pos == m_position) { // deleting only element in list
                pos = NULL;
            }
        }
        GetEntry(pos); // load data from new current position
        CStudent* ps = m_pList->GetAt(m_position); // remember so ca delete later
        m_pList->RemoveAt(m_position); // remove from list
        delete ps; // prevent memory leak
        m_position = pos; // set new position
        GetDocument()->SetModifiedFlag();
        GetDocument()->UpdateAllViews(this);
    }
	
}

Navigating in the List

Message handlers for navigation buttons/menu items:

void CStudentView::OnStudentHome() 
{
    // need to deal with list empty condition
    if (!m_pList->IsEmpty()) {
        m_position = m_pList->GetHeadPosition();
        GetEntry(m_position);
    }
	
}

void CStudentView::OnStudentEnd() 
{
    if (!m_pList->IsEmpty()) {
        m_position = m_pList->GetTailPosition();
        GetEntry(m_position);
    }
	
}

void CStudentView::OnStudentPrev() 
{
    POSITION pos;
    if ((pos = m_position) != NULL) {
        m_pList->GetPrev(pos);
        if (pos) {
            GetEntry(pos);
            m_position = pos;
        }
    }
	
}

void CStudentView::OnStudentNext() 
{
    POSITION pos;
    if ((pos = m_position) != NULL) {
        m_pList->GetNext(pos);
        if (pos) {
            GetEntry(pos);
            m_position = pos;
        }
    }
	
}

void CStudentView::ClearEntry()
{
    m_strName = "";
    m_nGrade = 0;
    UpdateData(FALSE); // invok DDX to clear form elements
    ((CDialog*) this)->GotoDlgCtrl(GetDlgItem(IDC_NAME));
// set focus to dialog item whose id is IDC_NAME
}

Hooking up the Navigation ToolBar

• Handled by the main window object, in the header file

  protected:  // control bar embedded members
  	CStatusBar  m_wndStatusBar;
  	CToolBar    m_wndToolBar;
  

Toolbar/Status created here

int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
	if (CFrameWnd::OnCreate(lpCreateStruct) == -1)
		return -1;
	
	if (!m_wndToolBar.CreateEx(this, TBSTYLE_FLAT, WS_CHILD | WS_VISIBLE | CBRS_TOP
		| CBRS_GRIPPER | CBRS_TOOLTIPS | CBRS_FLYBY | CBRS_SIZE_DYNAMIC) ||
		!m_wndToolBar.LoadToolBar(IDR_MAINFRAME))
	{
		return -1;      // fail to create
	}

	if (!m_wndStatusBar.Create(this) ||
		!m_wndStatusBar.SetIndicators(indicators,
		  sizeof(indicators)/sizeof(UINT)))
	{
		return -1;      // fail to create
	}

	// TODO: Delete these three lines if you don't want the toolbar to
	//  be dockable
	m_wndToolBar.EnableDocking(CBRS_ALIGN_ANY); 
	EnableDocking(CBRS_ALIGN_ANY);
	DockControlBar(&m_wndToolBar);

	return 0;
}

• Toolbar created by resource editor is loaded now (show - status/popup hint)
• This makes is dockable (demo)
  

Serializing

• When using CObList - call its serialize

  • It will call serialize of each object using polymorphism to call correct one

  • It also marks length of list somehow so you can store other objects after list and can read in OK

void CStudentDoc::Serialize(CArchive& ar)
{
    m_studentList.Serialize(ar);
}

• Add Serialize override to CStudent object

void CStudent::Serialize(CArchive& ar)
{
    if (ar.IsStoring()) {
        ar << m_strName << m_nGrade;
    }
    else {
        ar >> m_strName >> m_nGrade;
    }
}

Copyright chris wild 1999/2000.
For problems or questions regarding this web contact [Dr. Wild].
Last updated: February 02, 2000.